FIXED key shortcut to reset image interactor LUT modification

When the user Left Click and move the mouse in a slice viewer
(e.g., the axial slice), the LUT is modified interactively thanks
to the vtkInteractorStyleImage. This is very handy but there
was no simple way to reset the modifications applied to the LUT.
The user can now press "R" to reset these modifications.
Also fixed by this commit: the colors of the What's This panel
made it difficult to read. Text background and foreground colors
are now more constrasted and it should be easy to read the what's
this help.